Lithium ion secondary batteries, which feature small size and large capacity, have been widely used as power supplies for electronic devices such as mobile phones and notebook computers and have contributed to enhancing convenience of mobile IT devices. In recent years, larger-scale applications, such as power supplies for driving motorcycles and automobiles and storage cells for smart grids, have attracted attention.
In addition to a further enhancement in the energy density and the lifetime characteristics for endurance over long-term use, high safety under wide-range temperature conditions is required. Accordingly, materials have been widely contemplated for their electrolyte solution composition and electrode composition, which particularly greatly influences long-term cycles and safety.
As a method to enhance the cycle characteristics, an example of incorporation of a sulfate group in a positive electrode active material is disclosed. For example, Patent Literatures 1 and 2 disclose examples of a positive electrode active material containing a sulfate group in a lithium-transition metal composite oxide having a layered crystalline structure represented by LiMO2 (M is Co or Ni). Patent Literature 3 discloses an example of a lithium-manganese composite oxide having a spinel-type crystalline structure, wherein the composite oxide contains 0.16 to 1% by weight of a sulfate group. Patent Literature 4 discloses a lithium-nickel-manganese composite oxide for a positive electrode material, wherein the composite oxide contains a sulfur component consisting of a sulfate compound. These literatures suggest that the incorporation of the sulfate group in the positive electrode facilitates the passage of electrons around particles and enhances the cycle characteristics and load characteristics. These examples, however, have identified only the positive electrode material and the sulfate group, and have referred to no specific electrolyte solution.
As an electrolyte solution of lithium ion batteries, carbonate-based non-aqueous solvents are generally used. This is because carbonate-based solvents have excellent electrochemical resistance and are low cost. In most cases, a mixed electrolyte solution in which a cyclic carbonate such as ethylene carbonate (EC) and propylene carbonate (PC) is mixed with a chain carbonate such as diethyl carbonate (DEC) and dimethyl carbonate (DMC) is used. Cyclic carbonates have an effect of dissolving/dissociating lithium salts such as LiPF6 because of their high dielectric constant, and chain carbonates have an effect of increasing diffusion of lithium ions in an electrolyte solution because of their low viscosity.
In addition, it is known to use a sulfone compound having SO2 group as a solvent of an electrolyte solution. Sulfone compounds, which have a relatively high dielectric constant, can be used as a solvent having a high dielectric constant instead of the cyclic carbonate. For example, Patent Literature 5 discloses an electrolyte solution containing a lithium salt dissolved in a mixed solvent consisting of 20 to 80% by volume of sulfolane, 10 to 70% by volume of a low-viscosity organic solvent, and 10 to 30% by volume of an organic solvent having a high dielectric constant. Sulfone compounds, which have more excellent oxidation resistance than carbonate solvents, can suppress decomposition of the electrolyte solution under a high voltage as well as can reduce liquid shortage, increases in the internal pressure, and deformation and breakage caused by the increases in the internal pressure or the like.